Polyaxial bone anchoring device with enlarged pivot angle

ABSTRACT

A bone anchoring device includes a bone anchoring element with a spherical segment-shaped head, a receiving part configured to couple a rod to the bone anchoring element, and having a top end, a bottom end, a first passage at the bottom end with a first longitudinal axis, and a second passage at the top end with a second longitudinal axis that intersects the first longitudinal axis, a compression element, and a locking element. The receiving part also has a recess for the rod, the recess extending from the top end towards the bottom end and having a bottom that extends along an axis that is perpendicular to the second longitudinal axis. The receiving part or the compression element further has a seat. The compression element is configured to move along the first longitudinal axis while the locking element is configured to move along the second longitudinal axis.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.16/107,387, filed Aug. 21, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/008,273, filed Jan. 27, 2016, now U.S. Pat. No.10,064,659, which is a continuation of U.S. patent application Ser. No.13/948,700, filed Jul. 23, 2013, now U.S. Pat. No. 9,277,942, whichclaims priority to and the benefit of U.S. Provisional PatentApplication Ser. No. 61/676,485, filed Jul. 27, 2012, the contents ofwhich are hereby incorporated by reference in their entirety, and claimspriority from European Patent Application EP 12 178 289.0, filed Jul.27, 2012, the contents of which are hereby incorporated by reference intheir entirety.

BACKGROUND Field of the Invention

The invention relates to a polyaxial bone anchoring device providing apivot angle that is enlarged to one side compared to an opposite side.The polyaxial bone anchoring device comprises a bone anchoring elementwith a head and a shank and a receiving part for receiving a rod forcoupling the rod to the bone anchoring element. The receiving part hastwo passages, each coaxial with a longitudinal axis, wherein thelongitudinal axes of the two passages intersect one another and whereinone of the passages includes a seat configured to allow pivoting of thehead. The head is insertable into the seat from a bottom end of thereceiving part.

Description of Related Art

A polyaxial bone anchoring device with an enlarged pivot angle to oneside is known from U.S. Pat. No. 6,736,820 B2. This bone anchoringdevice comprises a screw member that is pivotably held in a receivingpart, wherein the receiving part has an open first bore and a secondbore on the end opposite to the first bore. On the bottom of the firstbore, a seat for the head is provided. In order that the screw membercan be pivoted to at least one side by an enlarged angle, an edgebounding the free end of the second bore viewed relative to the axis ofthe first bore is of asymmetric construction. The diameter of the secondbore is greater than that of a threaded section of the screw member andsmaller than that of the head.

Another bone anchoring device with an enlarged pivot angle is known fromU.S. Pat. No. 6,974,460 B2. It includes a coupling element having aninner surface defining a first bore coaxial with a first longitudinalaxis, and a second bore coaxial with a second longitudinal axis, wherebythe second longitudinal axis intersects the first longitudinal axis. Thecoupling element has a seat adjacent to the lower end of the couplingelement for the head of a bone anchoring element.

The polyaxial bone anchoring devices described above are top-loadinganchoring devices wherein the anchoring element is inserted into thereceiving part from an upper end thereof.

A bottom-loading polyaxial bone anchoring device is known from US2011/0276098 A1. This polyaxial anchoring device comprises a rodreceiving portion and a head receiving portion and an outer locking ringthat is configured to compress the head receiving portion to clamp andfinally lock the head. The anchoring element is insertable into thereceiving part from a bottom end of the receiving part.

SUMMARY

It is the object of the invention to provide a polyaxial bone anchoringdevice that provides for enlarged angulation and that provides for anincreased variety of applications.

A polyaxial bone anchoring device according to embodiments of theinvention is a bottom loading polyaxial bone anchoring device, whereinthe anchoring element can be inserted into a receiving part from abottom end of the receiving part. This allows for providing a modularsystem. In one embodiment, the bone anchoring device may be delivered bya manufacturer as a receiving part pre-assembled with an innercompression member, and separate therefrom, one or a plurality ofdifferent bone anchoring elements. In another embodiment, the boneanchoring device may be delivered by a manufacturer as a receiving partpre-assembled with an outer locking ring and also separate therefrom oneor a plurality of bone anchoring elements. By means of this, variousbone anchoring elements with shanks having different diameters, threadforms and/or various other different features can be combined with areceiving part according to actual clinical requirements in a particularclinical situation. This gives a surgeon or other practitioner a morediverse or wider choice of implants.

In addition, due to such modularity, the costs of stock-holding can bedecreased.

The polyaxial bone anchoring device provides for an enlarged pivot angleto one side compared to an opposite side, i.e. has a favored angledesign. A pull-out force that is necessary to pull out the boneanchoring element through an opening at the bottom end of the receivingpart is comparable to or at least not less than a pull-out force ofcomparable polyaxial bone anchoring devices that do not have a favoredangle design. This renders the bone anchoring device particularlysuitable for applications of lateral mass fixation, for example, at thecervical spine.

The polyaxial bone anchoring device does not require additional partscompared to anchoring devices without a favored angle design. Hence, thepolyaxial bone anchoring device provides for reduced dimensions in termsof height as well as in terms of diameter, which makes it particularlysuitable for applications where small-sized anchoring devices arerequired, such as in the fields of cervical spine surgery or pediatricapplications, trauma, and minimally open or minimally invasiveapplications for bone surgery.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages will become apparent from thedescription of embodiments by means of accompanying drawings. In thedrawings:

FIG. 1 shows an exploded perspective view of a polyaxial bone anchoringdevice according to a first embodiment.

FIG. 2 shows a perspective view of the polyaxial bone anchoring deviceof FIG. 1 in an assembled state.

FIG. 3 shows a perspective view of the receiving part of the polyaxialbone anchoring device of FIG. 1.

FIG. 4 shows another perspective view from the bottom of the receivingpart of FIG. 3.

FIG. 5 shows a top view of the receiving part of FIG. 3.

FIG. 6 shows a cross-sectional view of the receiving part along line A-Ain FIG. 5.

FIG. 7 shows a perspective view from the top of a compression member ofthe polyaxial bone anchoring device according to the first embodiment.

FIG. 8 shows another perspective view from the bottom of the compressionmember of FIG. 7.

FIG. 9 shows a top view of the compression member of FIG. 7.

FIG. 10 shows a cross-sectional view of the compression member alongline B-B in FIG. 9.

FIGS. 11 to 14 show cross-sectional views of steps of mounting thecompression member and the bone anchoring element to the receiving partaccording to the first embodiment, the section taken along line A-A inFIG. 5.

FIG. 15 shows a cross-sectional view of the polyaxial bone anchoringdevice in an assembled state, the section taken along line A-A of thereceiving part shown in FIG. 5, wherein the bone anchoring element andthe receiving part are pivoted relative to one another with a maximumpivot angle in a first direction.

FIG. 16 shows a cross-sectional view of the polyaxial bone anchoringdevice in an assembled state, the section taken along line A-A of thereceiving part shown in FIG. 5, wherein the bone anchoring element andthe receiving part are pivoted relative to one another with a maximumpivot angle in a second direction opposite to the first direction.

FIG. 17 shows a perspective view of a receiving part according to amodified first embodiment.

FIG. 18 shows a cross-sectional view of the receiving part of FIG. 17,the section taken in a plane perpendicular to the rod axis and goingthrough the middle of a channel for the rod.

FIGS. 19 to 21 show cross-sectional views of steps of mounting thecompression member to the receiving part according to the modified firstembodiment.

FIG. 22 shows an exploded perspective view of a polyaxial bone anchoringdevice according to a second embodiment.

FIG. 23 shows a perspective view of the polyaxial bone anchoring deviceof FIG. 22 in an assembled state.

FIG. 24 shows a perspective view of the receiving part of the polyaxialbone anchoring device according to the second embodiment.

FIG. 25 shows another perspective view from the bottom of the receivingpart according to the second embodiment.

FIG. 26 shows a top view of the receiving part according to the secondembodiment.

FIG. 27 shows a cross-sectional view of the receiving part according tothe second embodiment along line C-C in FIG. 26.

FIG. 28 shows a perspective view of a locking ring of the polyaxial boneanchoring device according to the second embodiment.

FIG. 29 shows another perspective view from the bottom of the lockingring of FIG. 28.

FIG. 30 shows a side view of the locking ring of FIG. 28.

FIG. 31 shows a top view of the locking ring of FIG. 28.

FIG. 32 shows a cross-sectional view of the locking ring along line D-Din FIG. 31.

FIGS. 33 to 36 show cross-sectional views of steps of assembling thepolyaxial bone anchoring device according to the second embodiment, thesection taken along line C-C in FIG. 26.

FIG. 37 shows a cross-sectional view of the polyaxial bone anchoringdevice according to the second embodiment in an assembled state, whereinthe bone anchoring element and the receiving part are pivoted relativeto one another with a maximum pivot angle in a first direction.

FIG. 38 shows a cross-sectional view of the polyaxial bone anchoringdevice according to the second embodiment in an assembled state, whereinthe bone anchoring element and the receiving part are pivoted relativeto one another with a maximum pivot angle in a second direction oppositeto the first direction.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a polyaxial bone anchoring device accordingto a first embodiment includes a bone anchoring element 1 in the form ofa bone screw having a threaded shank 2 and a spherical segment-shapedhead 3. The head 3 has a recess 4 for engagement with a tool. Thespherical segment-shaped head 3 may comprise a section including alargest diameter E of the head 3. The bone anchoring device furtherincludes a receiving part 5 for receiving a rod 100 to connect the rod100 to the bone anchoring element 1. In the receiving part 5, acompression member 6 for exerting pressure onto the head 3 is arranged.Furthermore, a locking element, in the form of, for example, an innerscrew 7 is provided for securing and fixing the rod 100 and for lockingthe head 3 in the receiving part 5.

Referring further to FIGS. 3 to 6, the receiving part 5 has a top end 5a and a bottom end 5 b. An outer surface of the receiving part issubstantially cylindrical adjacent to the top end 5 a and extends to atapering surface portion 50 b adjacent to the bottom end 5 b. A firstpassage 51 that is coaxial with a first longitudinal axis 51 a extendsfrom the bottom end 5 b toward the top end 5 a. A second passage 56coaxial with a second longitudinal axis 56 a extends from the top end 5a toward the bottom end 5 b. As can be seen in particular in FIGS. 5 and6, the first passage 51 and the second passage 56 are in communicationwith one another. The first longitudinal axis 51 a and the secondlongitudinal axis 56 a include an angle α with one another and intersectat a point in the interior of the receiving part 5. By means of thisdesign, the bottom end 5 b defines a first plane 51 b and the top end 5a defines a second plane 56 b, wherein the first plane 51 b and thesecond plane 56 b intersect. The angle α included by the firstlongitudinal axis 51 a and the second longitudinal axis 56 a may bebetween around 5° and around 20°, for example, approximately 15°.

The passages are now described in more detail. The first passage 51provides for an opening 52 at the bottom end 5 b, wherein a diameter ofthe opening 52 is greater than the greatest diameter E of the head 3.Adjacent to the bottom end 5 b the first passage 51 comprises a taperedsection 53 that narrows toward the bottom end 5 b. Following the taperedsection 53 an enlarged section 54 is provided with an inner diameterthat is greater than an inner diameter of the tapered section 53 andthat provides a space so that the compression member 6 can expandtherein. Following the enlarged section 54, there is a cylindricalsection 55 with a smaller diameter compared to the enlarged section 54,the end of which provides for a stop 55 a that limits an upward movementof the compression member 6.

The second passage 56 comprises adjacent to the top end 5 a a section 57with an internal thread that operates with the thread of the inner screw7. Following the threaded section 57, there is a section 58 with areduced inner diameter compared to the section 57 and the cylindricalsection 55 of the first passage, thereby providing the stop 55 a.Furthermore, adjacent to the top end 5 a, a substantially U-shapedrecess 59 is provided that extends into the direction of the bottom end5 b. By means of the U-shaped recess 59, a channel for receiving the rod100 is provided. The channel axis L extends perpendicular to the secondlongitudinal axis 56 a.

The receiving part 5 may further have inclined cuts 105 on either end ofthe channel formed by the U-shaped recess 59 that reduce the outerdiameter of the receiving part 5 in a direction of the channel axis L.Furthermore, tool engagement recesses 106 may be provided on either endof the channel and on both sidewalls of the channel. The channel axis Land the second longitudinal axis 56 a define a plane. The firstlongitudinal axis 51 a extends at an angle relative to that plane, i.e.intersects the plane. Therefore, an asymmetry in view of the pivot angleof the bone anchoring element is provided transverse to the channel L,which is transverse to the rod axis.

Referring further to FIGS. 7 to 10, the compression member 6 has a topend 6 a and a bottom end 6 b. Adjacent to the top end 6 a, there is asubstantially cylindrical portion 61 with a flat surface at the top end6 a and with an outer diameter that is only slightly smaller than aninner diameter of the section 55 of the first passage 51 so that thecylindrical section 61 fits into the cylindrical section 55 of thepassage and can move therein in axial direction of the firstlongitudinal axis 51 a.

Following the cylindrical portion 61, the compression member 6 has acap-like portion 62 that is recessed with respect to the cylindricalportion 61 at a position adjacent to the cylindrical portion 61. Thecap-like portion 62 defines a hollow interior 63 forming a seat for thehead 3 of the bone anchoring element 1. The hollow interior 63 issubstantially spherically-shaped mating the spherical shape of the head3. The second end 6 b of the compression member has an opening 64 forthe insertion of the head 3. An outer wall of the cap-like portion 62comprises a first section 62 a that is spherical and a second section 62b adjacent to the opening 64 that is tapered. The tapered section 62 bof the compression member 6 cooperates with the tapered section 53 ofthe first passage 51 of the receiving part 5. The cap-like portion 62 ofthe compression member further comprises a plurality of slits 65extending from an edge of the opening 64 through the cap-like portion upto a distance from the cylindrical portion 61. The respective ends 65 aof the slits may have the shape of a section of a circle or may beotherwise enlarged with respect to the dimension of the slits 65 tofacilitate a radial expansion or compression of the compression member.The dimension of the hollow interior 63, the number and dimensions ofthe slits 65 are such that the wall of the cap-like portion 62 isflexible enough to snap onto the head 3 when the head 3 is beinginserted. The seat provided by the cap-like portion 62 of thecompression member 6 allows pivoting of the head 3 in a symmetricalfashion relative to a central axis of the compression member 6.

A maximum outer diameter of the cap-like portion 62 can be slightlylarger than the inner diameter of the lower opening 52 of the receivingpart 5 at the bottom end 5 b, because the cap-like section 62 can becompressed during insertion of the compression member 6 into the firstpassage 51 through the lower opening 52. A length in axial direction ofthe compression member 6 is such that when the compression member 6 isinserted into the first passage 51 of the receiving part 5 and when itabuts with the top end 6 a against the stop 55 a of the receiving part5, the second end 6 b is located above the tapered section 53 of thefirst passage 51 of the receiving part (FIGS. 13 and 14) to allow aradial expansion of the cap-like portion 62. Because the compressionmember 6 is symmetric with respect to the pivoting of the head 3,securing of the compression member 6 against rotation is not necessary.

The compression member 6 further comprises a coaxial bore 66 forproviding access to the screw head 3 by a tool.

The inner screw 7 has a thread corresponding to the internal threadprovided in the second passage 56. Preferably, a thread form thatprevents the walls of the channel from splaying is used. Such a threadform can be, for example, a flat thread, a negative angle thread or asaw-tooth thread.

All parts described before may be made of a bio-compatible material,such as a bio-compatible metal like stainless steel or titanium, or abio-compatible metal alloy, for example, Nitinol, or are made of abio-compatible plastic material, for example, polyetheretherketone(PEEK). The parts may be of the same or of different materials.

The steps of assembling the bone anchoring device are now explained withrespect to FIGS. 11 to 14. First, as shown in FIGS. 11 and 12, thecompression member 6 is introduced from the bottom end 5 b through theopening 52 into the first passage 51 of the receiving part 5. When thecap-like portion 62 of the compression member 6 passes the taperedsection 53 of the first passage 57, it is slightly compressed. Thecompression member is further moved upward in the first passage 51toward the first end 5 a of the receiving part 5 until it abuts againstthe stop 55 a of the cylindrical section 55. In this position, thebottom end 6 b of the compression member 6 is above the tapered section53 of the first passage 51 and is located in the enlarged section 54that provides space for a radial expansion of the cap-like portion 62.

When the compression member 6 is in its uppermost position at the stop55 a, the head 3 of the bone anchoring element 1 is inserted into thefirst passage 51 of the receiving part 5 through the lower opening 52 atthe bottom end 5 b. When the head 3 enters into the hollow interior 63through the lower opening 64 of the compression member 6, the cap-likeportion 62 expands radially thereby permitting the insertion of the head3, as shown in FIG. 13. When the head 3 has entered the hollow interior63, the tapered outer surface section 62 b of the cap-like portion 62encompasses the head 3 at its greatest diameter E.

The use of the polyaxial bone anchoring device is now described withreference to FIGS. 15 and 16. The receiving part 5 may be delivered in apre-assembled manner with the compression member 6 inserted into thefirst passage 51. After assembling the head 3 of the anchoring element 1with the receiving part 5, the anchoring element 1 is inserted into abone part or a vertebra. Typically, at least two bone anchoring devicesare used that are connected with a rod 100. After insertion of theanchoring elements into the bone or into vertebrae, the receiving parts5 are aligned to allow the insertion of the rod 100 by pivoting themwith respect to the bone anchoring elements 1. The angular position maybe held in a preliminary manner by friction between the head 3 and thecap-like portion 62, when the compression member 6 is moved downward sothat its outer tapered section 62 b engages the tapered section 53 ofthe first passage 51. After insertion of the rod 100, the inner screw 7is inserted and tightened thereby moving the compression member 6further downward so that it is compressed around the head 3 to lock thehead 3 and the rod 100.

As can be seen in FIG. 15, in the assembled state, there is a firstmaximum pivot angle β₁ relative to the second longitudinal axis 56 a,which is defined by the abutment of the shank 2 of the bone anchoringelement 1 against the lower end 5 b of the receiving part in onedirection.

As can be seen in FIG. 16, a second maximum pivot angle (32 relative tothe second longitudinal axis 56 a is defined by the abutment of theshank 2 against the lower end 5 b in an opposite direction. The firstmaximum pivot angle β₁ is greater than the second maximum pivot angle(32. While the pivoting of the bone anchoring element 1 relative to thereceiving part 5 is symmetrical relative to the first longitudinal axis51 a, the pivoting is asymmetrical relative to the second longitudinalaxis 56 a, and is therefore also asymmetrical relative to the channelaxis L and the rod 100.

Because the seat for the head 3 provided by the compression member 6 issymmetrical with respect to the pivoting of the head 3, the pull-outforce for the head 3 is not decreased compared to comparable polyaxialbone anchoring devices without an enlarged pivot angle.

The polyaxial anchoring device may be particularly useful inapplications of cervical spine surgery, wherein enlarged pivot anglesonly to one side and to an opposite side may be needed.

A modification of the first embodiment of the polyaxial bone anchoringdevice is shown in FIGS. 17 to 21. The modified first embodiment differsin the shape of the receiving part and is identical or highly similar tothe first embodiment in all other parts. The receiving part 5′ has asecond passage 56′ with a diameter greater than the outer diameter ofthe cylindrical portion 61 of the compression member 6. The secondpassage 56′ extends into the cylindrical section 55 of the first passage51. Therefore, as can be seen in particular in FIGS. 18 and 19, theinner diameter of the second passage 56′ is sized such as to allow theintroduction of the compression member 6 from the top end 5 a.

Steps of assembling the receiving part 5′ and the inner compressionmember 6 are shown in FIGS. 19 to 21. As depicted in FIG. 19, thecompression member 6 is introduced into the first passage 56′ in anorientation in which the opening 64 shows toward the bottom end 5 b ofthe receiving part 5′. When the compression member 6 has reached thecylindrical section 55 of the first passage 51, the compression member 6is tilted to align it with the first longitudinal axis 51 a so that itcan engage the cylindrical section 55 of the first passage 51 as can beseen in FIG. 20. As shown in FIG. 21, the compression member 6 canfurther be moved downward until its tapered outer section 62 b engagesthe tapered section 53 of the receiving part 5′. The other steps are thesame as for the first embodiment.

In a still further modified embodiment, a transverse pin (not shown) canbe provided in the receiving part that cooperates with the compressionmember to secure the compression member against tilting once it hasreached its position in the first passage.

Alternatively, with the modified first embodiment, the compressionmember 6 can also be inserted from the bottom end 5 b of the receivingpart 5′.

A second embodiment the polyaxial bone anchoring device is now describedwith reference to FIGS. 22 to 38. As shown in particular in FIGS. 22 and23, a polyaxial bone anchoring device according to the second embodimentdiffers from the polyaxial bone anchoring device according to the firstembodiment in that instead of an inner compression member, an outerlocking ring 8 is used that is configured to compress a portion of areceiving part 500 to clamp and lock the head 3 therein. All portions ofthe second embodiment that are identical with the function or the designof corresponding portions of the first embodiment have the samereference numerals and the description thereof will not be repeated.

The receiving part 500 comprises a first portion 9 that is adjacent tothe bottom end 5 b and a second portion 10 that is adjacent to the topend 5 a. The first portion 9 comprises the first passage 51 coaxial withthe first longitudinal axis 51 a and the second portion 10 comprises thesecond passage 56 coaxial with the second longitudinal axis 56 a. As inthe first embodiment, the first and the second longitudinal axesintersect and include an angle α. The plane 51 b defined by the bottomend 5 b is inclined relative to the plane 56 b defined by the top end 5a and therefore, the planes intersect.

At the bottom end 5 b, the first portion 9 has an opening 91 thediameter of which is such that the head 3 can be inserted. At a distancefrom the bottom end 5 b, the first passage 51 comprises a head receivingportion 92 having a spherical segment shape that mates the shape of aportion of the head 3. The head receiving portion 92 provides a seat forthe head 3 that allows the head 3 to pivot therein. Furthermore, thehead receiving portion 92 is configured to encompass the head 3 of thebone anchoring element 1 laterally, thereby covering the largestdiameter E of the head 3.

A plurality of slits 93 are provided in the head receiving portion 92that are open to the second end 5 b. The slits 93 render the headreceiving portion 92 flexible so that it can be expanded when the head 3is inserted and can be compressed to clamp and finally lock the head 3by means of friction. The number and size of the slits 93 is provideddepending on the desired flexibility of the head receiving portion 92.

As can be seen in particular in FIG. 27, the head receiving portion isrotationally symmetrical with respect to the first longitudinal axis 51a. More in detail, when the head 3 is inserted into the head receivingportion 92, it can pivot to a maximum pivot angle in one direction andto the same maximum pivot angle in the opposite direction in relation tothe first longitudinal axis 51 a.

Following the head receiving portion 92, the first passage 51 furthercomprises a cylindrical section 94 extending to a distance from thebottom end 5 b. At least some of the slits 93 may extend into thecylindrical section 94 to increase the flexibility of the head receivingportion 92.

The outer surface of the first portion 9 of the receiving part 500comprises several sections. Adjacent to the bottom end 5 b there is acylindrical section 95. Following the cylindrical section 95 there is acircumferentially extending lower groove 96. Adjacent to the lowergroove 96 there is an outwardly curved or conically widening section 97,the diameter of which increases toward the bottom end 5 b. Adjacent toor at some distance from the outwardly curved section 97 there is acylindrical section 98 that is symmetrical in relation to the firstlongitudinal axis 51 a. An outer diameter of the cylindrical section 98is greater than an outer diameter of the outwardly curved section 97. Adownward facing surface of the cylindrical section 98 forms a stop 98 afor the locking ring 8. Adjacent to the cylindrical section 98, acircumferentially extending upper groove 99 is provided for engagementwith a portion of the locking ring 8. The upper groove 99 has an upperwall providing a stop 99 a for the locking ring 8.

The second portion 10 of the receiving part 500 comprises the secondpassage 56 that consists of a internally threaded section 101 adjacentto the top end 5 a. Furthermore, the second portion 10 comprises asubstantially U-shaped recess 102 extending from the top end 5 a towardthe bottom end 5 b into the first passage 51. Hence, the bottom of theU-shaped recess 102 reaches into the hollow cylindrical section 94 ofthe first passage 51.

An outer surface of the second portion 10 is substantially cylindricaland may continue in a small tapered section 103 at the transitionbetween the second portion 10 and the first portion 9. Further, as canbe seen in FIGS. 24 to 26 cut-outs 11 are provided in the cylindricalsection 98 of the first portion 9 on either end of the channel formed bythe U-shaped recess 102.

The locking ring 8 is now described referring to FIGS. 28 to 32. Thelocking ring 8 has an upper end 8 a and a lower end 8 b. In the mountedstate, the upper end 8 a is oriented in the direction of the top end 5 aof receiving part 500, while the lower end 8 b is oriented toward thebottom end 5 b of the receiving part 500. Near the lower end 8 b, afirst portion 81 with an inner surface 81 a is provided that cooperateswith the outwardly curved section 97 of the receiving part 500 tocompress the head receiving portion 92. The outer surface of the firstportion 81 may also be tapered to reduce an outer bottom diameter. Thesize of the first portion 81 is such that, for example, the taperedinner surface 81 a can engage the outwardly curved section 97 of thehead receiving portion 92 to exert a compression force onto the headreceiving portion 92. The inner surface 81 a of the first portion 81 ofthe locking ring 8 can also be curved with a curvature directed toward acenter of the locking ring 8.

At the lower end 8 b, the locking ring 8 includes an inwardly projectingedge 82, an inner diameter of which is smaller than an inner diameter ofthe other portions of the locking ring 8. The inwardly projecting edge82 is configured to engage the lower groove 96 of the receiving part500.

The locking ring 8 further has upwardly extending wall portions 83 athat are separated from each other by slits 84. The upwardly extendingwall portions 83 a are arranged at an outer circumference of an innercircumferential shoulder 85 of the locking ring 8, and render the upperportion of the locking ring flexible. The number and size of the slitsand the thickness of the wall portions 83 a are configured such that adesired flexibility is realized. At the free ends of the wall portions83 a engagement sections 83 b are provided that are shaped so as toengage the upper groove 99 provided on the outer surface of thereceiving part 500.

The locking ring 8 is sized in such a way with respect to the firstportion 9 of the receiving part 500, that the head receiving portion 92can expand within the locking ring 8 to allow the insertion of the head3 when the locking ring 8 is in the first position as shown in FIG. 34.

Two projections 86 that are located diametrically opposite to eachother, are formed between the flexible wall portions 83 a of the lockingring 8. The projections 86 have a height where they extend into thecut-outs 11 and project above the bottom of the U-shaped recess 102,when the locking ring 8 is in a position in which the head 3 is not yetlocked as shown in FIGS. 34 and 35. A free end surface 86 a of theprojections 86 is concave. For example, the free end surface 86 a can besubstantially V-shaped to provide two lines of contact with the rod 100.

The locking ring 8 is arranged in such a manner around the first portion9 of the receiving part 500 that the projections 86 are located atpositions of (e.g., are aligned with) the U-shaped recess 102. In thiscase, the projections 86 prevent the locking ring 8 from rotating whenthe rod 100 is not inserted.

The flexibility of the head receiving portion 92 and the size of thehead receiving portion allows for assembling the locking ring 8 from thebottom end 5 b onto the first portion 9.

Steps of assembling the polyaxial bone anchoring device are describedwith reference to FIGS. 33 to 36. In a first step, shown in FIG. 33, thelocking ring 8 is mounted onto the first portion 9 of the receiving part500 from the bottom end 5 b with its flexible wall portions 83 aoriented toward the top end 5 a. When mounting the locking ring 8, thehead receiving portion 92 is slightly compressed. As shown in FIG. 34,the locking ring 8 is moved upward until its circumferential shoulder 85abuts against the lower surface 98 a of the cylindrical outer surfacesection 98 of the first portion 9. The flexible wall portions 83 a arenot in engagement with the upper groove 99. The projections 86 extendinto the cut-outs 11 and project over the bottom of the U-shaped recess102. The locking ring 8 is held in a preliminary manner in this positionbecause the inwardly projecting edge 82 engages the lower groove 96.

As shown in FIG. 35, the head 3 of the bone anchoring element 1 can beinserted into the head receiving portion 92 of the first portion 9 whenthe locking ring 8 is in the position descibed before.

In a next step (not shown), the locking ring 8 may be moved downwardtoward the bottom end 5 b until its flexible wall portions 83 aresiliently snap into the upper groove 99. Once in this position, thefree upper edge of the engagement portions 83 b abut against the stop 99a of the groove 99, thereby preventing upward movement of the lockingring 8 out of this position. In this configuration, the head 3 isprevented from removal through the lower opening 91. Furthermore, inthis configuration, the head 3 may be clamped by friction so that acertain angular position may be maintained in a preliminary manner.

In a next step, as shown in FIG. 36, the rod 100 is inserted into thechannel from the top end 5 a until it abuts against the free surface 86a of the projections 86. Because the first and second longitudinal axesare tilted with respect to one another, the engagement of the rod 100with the free end surface 86 a is slightly asymmetric. If the free endsurface 86 a provides for a contact along two lines, for example with aV-shaped cross-section, a safe contact can be achieved even in suchasymmetric engagement condition.

Once the inner screw 7 is inserted into the receiving part 500 andtightened, the force exerted by the rod 100 onto the projections 86 ofthe locking ring 8 move the locking ring 8 downward into a finalposition in which the head receiving portion 92 is compressed so as tolock the head 3.

FIGS. 37 and 38 show an assembled configuration, wherein the boneanchoring element 1 is pivoted relative to the receiving part 500 at amaximum pivot angle β₁ to one side (FIG. 37) and a maximum pivot angle(32 to the opposite side (FIG. 38). The maximum pivot angle is definedby the abutment of the shank 2 against the lower portion of the seat.The maximum pivot angle β₁ in FIG. 37 is larger than the maximum pivotangle (32 in FIG. 38 because of the inclination of the first passage 51of the receiving part 500 with respect to the second passage 56.

Modifications of this embodiment are possible. The locking ring can haveany other shape that fulfills the function of compressing the headreceiving portion. The surface of the free end of the projections can beflat. Alternatively, the projections can be omitted as long as the rodcan engage the locking ring to move it downward. The outer surface ofthe first portion and the inner surface of the locking ring can haveother shapes that allow for compression of the first portion by means ofan increasing force when the locking ring is shifted downward relativeto the receiving part.

For both embodiments, the orientation of the first longitudinal axisrelative to the second longitudinal axis may be different from theorientation described before. For example, the first longitudinal axismay lie in the plane defined by the channel axis and the secondlongitudinal axis. The resulting pivoting of the bone anchoring elementis then asymmetric in a direction along the rod axis. The firstlongitudinal axis may also be angled at an acute angle with respect tosaid plane and with respect to the channel axis.

Furthermore, the seat may be substantially symmetrical with respect tothe pivoting of the head when the head pivots in any direction and theopposite direction thereof. Small cutouts at the lower edge of the seatmay be present. In addition, the head and the seat may be configured toallow a symmetric pivoting in one or more discrete planes only so thatthe bone anchoring device has a monoplanar construction with respect tothe pivot angles.

Modifications of both embodiments may further include the use ofspecific materials for rendering the compression member or the headreceiving portion flexible. Such materials can be used instead of oradditionally to providing slits.

For the bone anchoring element, various different kinds of anchoringelements can be used and combined with the receiving part. Theseanchoring elements may be, for example, screws with different length,screws with different diameters, cannulated screws, screws withdifferent thread forms, nails, hooks etc. For some anchoring elements,the head and the shaft may also be separate parts that are connectableto each other.

Other kinds of locking devices including outer nuts, outer caps, bayonetlocking devices, or others are also possible. Also, a two part lockingdevice may be used wherein one part locks the head and the other partlocks the rod.

It shall also be noted that portions of the different describedembodiments can also be combined with each other in various differentcombinations.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A polyaxial bone anchoring device comprising: a receiving part forreceiving a rod for coupling the rod to a head of a bone anchoringelement; the receiving part comprising a top end and a bottom end, afirst passage coaxial with a first longitudinal axis and a secondpassage coaxial with a second longitudinal axis, wherein the first andsecond longitudinal axes intersect; wherein the first passage includes aseat that opens toward the bottom end, the seat being configured toallow pivoting of the head relative to the seat.
 2. The polyaxial boneanchoring device of claim 1, wherein the seat is substantiallysymmetrical with respect to the pivoting of the head when the headpivots in any direction and in the respective opposite direction.
 3. Thepolyaxial bone anchoring device of claim 1, wherein the first passageextends from the bottom end toward the top end and wherein the secondpassage extends from the top end toward the bottom end.
 4. The polyaxialbone anchoring device of claim 1, wherein the first passage and thesecond passage are in communication with one other.
 5. The polyaxialbone anchoring device of claim 1, wherein the receiving part furthercomprises a channel for receiving the rod.
 6. The polyaxial boneanchoring device of claim 5, wherein the channel is formed by asubstantially U-shaped recess adjacent to the top end. 7-8. (canceled)9. The polyaxial bone anchoring device of claim 1, wherein the head hasa spherically-shaped outer surface portion including a greatest diameterof the sphere and wherein the seat has a substantially concave surfaceportion adapted to engage at least the outer surface portion of the headthat includes the greatest diameter.
 10. The polyaxial bone anchoringdevice of claim 1, further comprising a locking element engageable withthe receiving part for locking the position of the receiving part withrespect to the bone anchoring element.
 11. The polyaxial bone anchoringdevice of claim 10, wherein the locking element urges the rod toward thebottom end of the receiving part which in turn clamps the head of theanchoring element in the seat for locking the receiving part and theanchoring element from further movement relative to one another.
 12. Thepolyaxial bone anchoring device of claim 1, wherein the receiving parthas a single inner surface defining both the first and the secondpassage and wherein the seat is defined by an interior wall portion thefirst passage. 13-14. (canceled)
 15. The polyaxial bone anchoring deviceof claim 1, wherein the seat is provided in a separate compressionmember that is provided in the first passage.
 16. The polyaxial boneanchoring device of claim 15, wherein the compression member is adaptedto cover at least a portion of the head from an upper side of the head.17. The polyaxial bone anchoring device of claim 15, wherein thecompression member is movable in the first passage toward the top endagainst a stop.
 18. The polyaxial bone anchoring device of claim 15,wherein the first passage comprises a section with an enlarged diameterfor allowing the compression member to expand therein when the head isinserted.
 19. The polyaxial bone anchoring device of claim 1, whereinthe receiving part comprises an opening at the bottom end with adiameter that is greater than the greatest diameter of the head.
 20. Thepolyaxial bone anchoring device of claim 1, wherein the receiving partand the seat are adapted to allow pivoting of the bone anchoring elementin the seat in a symmetrical manner with respect to the firstlongitudinal axis.
 21. The polyaxial bone anchoring device of claim 1,wherein the second longitudinal axis is perpendicular to an axis of therod when the rod is received in the receiving part.